Gas separation membranes capable of selective carbon dioxide separation and recovery are currently applied to various processes, such as carbon dioxide capture before and after combustion in thermal power plants, removal of carbon dioxide present in off-gases from petrochemical processes and flue gases after combustion, and natural gas and biogas purification. Particularly, gas separation membranes based on polymeric materials are required to have not only excellent thermal, chemical, and mechanical properties but also high gas permeability and selectivity for their successful applications to practical industries. However, both the gas permeability and selectivity of polymeric material-based separation membranes are generally difficult to maintain at high levels because of their trade-off relationship. Thus, considerable research efforts have been directed to overcoming this difficulty.
Some conventional monolayer membranes are based on glassy polymeric materials having a high attractive force between the polymer chains do not use supports. Film formation processes for the manufacture of the monolayer membranes leave no defects (or pin holes) but the permeability of the monolayer membranes is disadvantageously low. In recent years, carbon membranes have been developed that are manufactured by final carbonization of monolayer membranes based on various polymeric materials. The carbon membranes are generally obtained by carbonization of polymer precursors in the form of films at high temperature. The high-temperature carbonization makes the carbon membranes microporous. The carbon membranes exhibit high gas permeability and selectivity and are advantageous in terms of long-term stability, durability, chemical resistance, and high-temperature stability. However, the mechanical properties (e.g., elasticity and tensile strength) of the carbon membranes are unsatisfactory and the manufacture of the carbon membranes requires a high temperature of 600 to 1,000° C. and a long time, incurring a considerable cost. Poor processability of the carbon membranes resulting from difficulty in thin film formation is an obstacle to the commercialization of the carbon membranes. Another serious problem of the carbon membranes is that defects may be formed during film formation processes. For example, carbon membranes manufactured by carbonization of hollow fiber membranes made of cellulose esters are known to have improved carbon dioxide permeability and selectivity for carbon dioxide over methane gas but are still insufficient in solving the above-mentioned problems, including defect formation, on account of their characteristics (Patent Document 1: Korean Patent Publication No. 2011-0033111).
Since it was reported that carbon nanotube films have high gas permeability and selectivity, unlike traditional monolayer membranes and carbon membranes, considerable research has been conducted on composite membranes in which carbon nanotubes are mixed inside a polymer matrix. For example, a composite membrane for gas separation is known in which single walled carbon nanotubes functionalized with long-chain alkyl amines are mixed inside a polysulfone matrix to facilitate dispersion of the carbon nanotubes in the polymer (Non-Patent Document 1: Sangil Kim et al., J. Membr. Sci. 294 (2007) 147-158). The composite membrane is also known to have improved carbon dioxide permeability compared to polysulfone monolayer membranes. However, the composite membrane exhibits lower selectivity for carbon dioxide over methane gas than polysulfone monolayer membranes, and as a result, the trade-off relationship between the gas permeability and selectivity of the composite membrane still remains unsolved to a satisfactory level.
Recent attention has been paid to graphene materials that have 2-dimensional planar monolayer structures, exhibit high mechanical strength and excellent thermal and chemical properties, and can be formed into thin films. For example, a composite membrane manufactured by the transfer of graphene to a porous polymer support is known (Patent Document 2: U.S. Patent Publication No. 2012-0255899). The use of the graphene-containing composite membrane enables the separation of oxygen from a nitrogen-oxygen mixed gas, leading to oxygen enrichment or nitrogen production. However, despite the expectation that the graphene thin film will improve the gas permeability and selectivity of the composite membrane, the formation of some defects on the membrane surface cannot be avoided, and as a result, the permeability of carbon dioxide is considerably lower than those of other gases, such as helium, hydrogen, oxygen, nitrogen, and methane.
A functionalized graphene-containing composite membrane is known which includes a film layer formed by coating a dispersion of functionalized graphene on an electrically non-conductive porous polymer support by vacuum filtration (Patent Document 3, International Patent Publication No. 2011-066332). The application of the composite membrane to a chemical sensor or an electrochemical double layer capacitor is also known in the patent document. However, the application of the composite membrane to a gas separation membrane is neither suggested nor indicated in the patent document. If the composite membrane is applied to a gas separation membrane, improved carbon dioxide permeability and selectivity will be expected but stability problems encountered in the ultrathin film structure of the coating layer bring about structural deformation of the composite membrane when long-term exposure to a particular gas, such as carbon dioxide, inevitably causing deterioration of permeability and selectivity.